Electron multiplier apparatus



Patented Feb. 1, 1944 UNITED STATES PATENT ELECTRON MULTIPLIER APPARATUSApplication June 16, 1941, Serial No. 398,342 In Great Britain March 6,1941 (Cl. 2,50F27) 7 Claims.

This invention relates to electron multiplier apparatus comprising anelectron multiplier having an electrode system including a primarycathode, one or more secondary cathodes for secondary electron emissionand a collector, together with means for applying to the secondarycathode or cathodes and the collector, positive potentials with respectto the primary cathode of progressively increasing magnitude. Inoperation, electrons emitted by the primary cathode impinge on asecondary cathode and liberate other electrons which impinge on the nextsecondary cathode, when more than one, and so on in succession to thecollector. The present invention is concerned with electron multipliersemploying secondary cathodes which are in the form of a Wire grid ormesh or perforated plate or are of any other construction providing asurface on which approaching electrons may impinge and an aperture orapertures through which the secondary electrons may pass to a succeedingelectrode. With such secondary cathodes some of the approachingelectrons may pass through the apertures without producing secondaryemission but if the apertures are decreased in size to minimise thisaction then there is the disadvantage that the eld of the next followingelectrode does not penetrate the apertures so Well for the purpose ofaccelerating the secondary electrons. The object of the presentinvention is to minimize this disadvantage.

According to the present invention there is provided an electronmultiplier apparatus comprising an electron multiplier having asecondary cathode with apertures for the passage of electrons and anadditional secondary electrode emitting electrode spaced from thesecondary cathode and also having apertures for the passage of electronsand means for applying to the secondary cathode and the additionalelectron emitting electrode positive potentials of the same order ofmagnitude. With this arrangement the provision for secondary emission isincreased without reducing the aperture size and consequently thepenetration by the field of the next following electrode. In onearrangement in which the additional electrode is disposed in front ofthe secondary cathode in the direction of approach of electrons thepositive potential applied to the l additional electrode is higher thanthat applied to the secondary cathode 4but substantially less than thatapplied to the next following electrode.

A further feature of the invention consists in that the additionalsecondaryA electron emitting electrode is connected by a resistance orother impedance to means for applying to it a potential greater thanthat applied to the following secondary cathode.

The invention also comprises the construction of electron multiplierhaving one or more secondary cathodes consisting of two or more electronemissive grids; perforated plates or like members spaced apart andelectrically interconnected within the multiplier. In a preferredconstruction the secondary cathode consists of tWo or more perforatedplates spaced apart and arranged with the perforations of the plates outof line.

Specific embodiments of the invention are illustrated diagrammaticallyand by way of eX- ample in the accompanying drawing, in which- Figures1, 2 and 3 are diagrams of diiferent forms of electron multiplieraccording to the invention, and

Figure 4 is an enlarged detail View of a construction of secondarycathode.

Referring to Figure 1, there is shown an electron multiplier Illcomprising a thermionic primary cathode I I, together with a controlgrid I 2 and an accelerator E3 for the primary electrons. In addition,there is a plurality of secondary cathodes I4 each in the form of agrid, perforated plate or like apertured member, a final secondarycathode I5 in the form of an imperforate plate and a grid-like collectorI6. Positive potentials with respect to the cathode I I are applied tothe accelerator, the secondary cathodes and the collector fromappropriate tappings on a potential divider I l across Which an electricsupply indicated at I8 is connected. .A signal input device I9 isconnected to the control grid I2 and the cathode I I and an outputdevice 20 is connected to the collector electrode I6.

In front of each of the secondary cathodes I4 there is an additionalelectrode 2| which, like the cathode I4, is sensitised for secondaryemission and is in the form of a grid or perforated plate or the like.Each electrode 2I is connected to the potential divider so as to have apotential somewhat higher than the secondary cathode I4 immediatelyfollowing it but substantially lower than the potential of the nextsecondary cathode I4. For instance, if the potential difference fbetweentwo secondary cathodes I4 -is 300 volts the potential of the electrode2l in front of the first of these secondary cathodes may be of the orderof 20 volts positive thereto.

In the arrangement shown in Figure 2 each secondary cathode Ill andassociated additional electrode 2| are joined together Within themultiplier so as to have the same positive potential derived from thepotential divider I'I.

Each of the secondary cathodes I4 and the additional electrode 2| infront of it are preferably in the form of perforated plates, forexample, as described in the specification of my U. S. Patent No.2,254,128, and in the preferred arrangement of the electrodes accordingto this invention the plates are disposed with the apertures of the twoplates in staggered relationship. For example,

ode I4 and the additional electrode 2I, each consisting of a perforatedplate, are joined together to form a unitary electrode. The apertures inthe two plates are staggered.

With the secondary cathode arrangement according to this invention someof the approaching electrons will pass through the apertures in theelectrode 2| and impinge on the electrode I4 to liberate secondaryelectrons. The remaining approaching electrons will impinge on theelectrode 2l and liberate secondary electrons. The accelerating eld ofthe next following electrode penetrates the apertures in the electrodeI4 for collecting or accelerating the secondary electrons emitted bythis electrode and at least the secondary electrons emitted at highvelocity by the associated electrode 2l. In the arrangement shown inFigure 3 each of the additional electrodes 2| is connected by means of aresistance 22 or other impedance with the second following secondaryAcathode I4 so as to have a nominal potential which is considerablyhigher than the secondary cathode'which immediately follows it. In thisway, a portion of the standing current of the multiplier is divertedthrough the resistance or other impedance with the result that theprimary electron-space current can be increased to a correspondingextent without increasing the standing current through the output of themultiplier. This resistance or impedance is preferably such that thedrop in potential across it, due to the diverted standing current,lowers the potential on the electrode 2| to a value approximately equalto or slightly higher than the potential applied to the followingsecondary cathode I4.

In a specific construction of the electrodes I4 and 2 I Vthey consist ofperforated plates, as shown in Figure 4, spaced apart preferably by adistance of l mm., which spacing may, however, be increased up to, say,2.7 mm. The plates have perforations of tapering form of which thesmallest diameter is from 1.2 to 1.5 mm. and there are approximately 14perforations per sq. cm. of plate. This construction is for an operatingvoltage of 300 volts per stage of the multiplier. It will be understood,however, that the invention is not restricted to this specificconstruction, and furthermore, that two or more additional electrodes 2lmay be provided in front of one another and for example, directlyelectrically connected with one another and with the secondary cathodeI4.

I claim:

l. An electron multiplier apparatus comprising the combination with anelectron multiplier having a primary cathode, a secondary cathode withapertures for the passage of electrons and a collector, an additionalsecondary electron emitting electrode spaced from the secondary cathodeand also having apertures for the passage of electrons of means forapplying to the said secondary cathode and the additional electronemitting electrode as shown in Figure 4, the mainV secondary cathv xedpositive potentials of the same order of magnitude.

2. An electron multiplier apparatus comprising the combination with anelectron multiplier having a primary cathode and a group of electrodescomprising at least one secondary cathode with apertures for the passageof electrons and a collector, means applying to the electrodes of saidgroup positive potentials of grogressively increasing value with respectto the primary cathode and an additional secondary electron emittingelectrode spaced from said secondary cathode to the front thereof in thedirection of approach of electrons of means for applying to saidadditional electrode a positive potential which is higher than thatapplied to said secondary cath- Y ode but is substantially less thanthat applied to the next following electrode of said group.

3. An electron multiplier apparatus comprising the combination with anelectron multiplier having a primary cathode and a group of electrodescomprising at least one secondary cathode with apertures for the passageof electronsfand a col-v lector, means applying to the electrodes ofsaid group positive potentials of progressively increasing value withrespect to the primary cathode and an additional secondary electronemitting electrode spaced from said secondary cathode to the frontthereof in the direction of approach of electrons, a resistanceconnected to said additional electrcde ofmeans for applying to saidadditional electrode through said resistance a potential which is higherthan that applied to the next following electrode of said group.

4. An electron multiplier apparatus comprising the combination with anelectron multiplier having a primary cathode and a group of electrodescomprising at least one secondary cathode with apertures for the passageof electrons and a col-- lector, means applying to the electrodes ofsaid group positive potentials of progressively increasing value withrespect to the primary cathode and an additional secondary electronemitting electrode spaced from said secondary cathode to the frontthereof in the direction of approach of electrons, an impedanceconnected to said additional electrode of means for applying to saidadditional electrode through said impedance a potential which is higherthan that applied to the next following electrode of said group.

5. An electron multiplier comprising a primary cathode, at least onesecondary cathode and a collector electrode, said secondary cathodeconsisting of at least two multi-apertured members spaced apart anddirectly connected with one another and both adapted to emit secondaryelectrons.

6. An electron multiplier comprising a primary cathode, at least onesecondary cathode consisting of at least two perforated plates spacedapart with the perforations of the plates out of line and electricallyinterconnected with one another and both adapted to emit secondaryelectrons, a collector electrode, and means for applying a fixed

